Modern communication systems support deployments wherein a single source terminal is configured to communicate various types of information to multiple terminals. One example of such a deployment includes a cellular communication system, such as a universal mobile telecommunication system (UMTS), wherein a base station communicates with a plurality of user terminals. Another example deployment includes an access point transmitting to multiple terminals in a wireless local area network (WLAN) system. These single-source-to-multiple-terminal types of deployments are commonly referred to as “broadcasting”, “multicasting”, or more generally as “point-to-multipoint” (PtM) communications.
Traditional approaches to deploying such PtM communication systems may be classified into one of two categories. In the first category, communication transmissions to the various user terminals are permitted to interfere with each other. An example of such a scheme includes a traditional code division multiple-access (CDMA) system, wherein pseudo-random codes are used to code various communications prior to their transmission. Although pseudo-random coding in the transmission source is useful in mitigating cross-interference, the true burden of mitigating this interference lies in the receiving terminals themselves.
In the second, more frequently utilized category of PtM deployments, interference is altogether avoided via the use of orthogonal transmissions. Examples of such deployments include frequency division multiple access (FDMA) systems and time division multiple access (TDMA) systems, such as for example, a Global System for Mobile Communications (GSM) standard system. In these types of PtM communication systems, a transmitting terminal transmits various communication signals using mathematically orthogonal or non-interfering “vectors” in multi-dimensional “signal spaces”. These vectors may be defined as a frequency range (as in a FDMA system) wherein the signal space dimensions (or axes) correspond to different frequencies; a time-slot (as in a TDMA system), wherein the signal space axes correspond to different timeslots; or a Walsh code (as in an orthogonal code CDMA system), wherein the signal space axes correspond to different orthogonal Walsh codes. Unlike the first category of PtM communication systems, the transmitting terminal in this second category is often solely responsible for preventing signal interference. As a result, receiving terminals in such systems are typically no more complex than those used in basic point-to-point communication systems. Although this category of PtM systems is superior in many aspects, such as in transmitter/receiver complexity and in the performance of communication links, it should be understood that the performance of such systems is limited by the number of available orthogonal spaces and/or dimensions.
Referring now to FIG. 1, a graph 100 illustrating achievable data transmission rates 101-105 to the two receivers, Rx A and Rx B, in a PtM system is shown. It should be understood that FIG. 1 is for illustrative purposes only and that is does not represent actual test results.
If all the available transmission bandwidth in a FIG. 1 system were allocated to say, receiver Rx A, Rx A would receive service at a highest achievable data rate C1 and Rx B would receive no data. Similarly, if all the available transmission bandwidth were allocated to receiver Rx B, Rx B would receive service at a highest achievable data rate C2 and Rx A would receive no data.
If receivers Rx A and Rx B were operating in, for example, a TDMA system (which is equivalent to time-sharing), they would be capable of achieving data transmission rates at and to the left of the solid line 101 on the graph 100. As time sharing represents a special case of orthogonal multiplexing, the same rates are achievable by any system which maintains orthogonality between transmission signals.
In a PtM system, where orthogonality between transmission signals is not maintained, the transmission performance to any number of receivers can suffer as compared to that of an orthogonal system, such as TDMA. To illustrate, reference is again made to FIG. 1. Line 102 may be representative of achievable data rates in a typical random code CDMA system utilizing a standard RAKE receiver. Line 103 may be representative of achievable data rates in a typical random code CDMA system utilizing a more advanced linear receiver, such as a linear MMSE multi-user detector. As indicated by the graph 100, the achievable data rates represented by line 103 are superior to those represented by line 102. Neither provides, however, the performance of line 101, which as described above, represents an achievable performance rate of transmission signals that are maintained orthogonal to each other.
It is well known from information theory that data rates superior to that of orthogonal coding (e.g., TDMA) are achievable in PtM systems. These superior data rates may be represented, for example, by lines 104 and 105 of the graph 100 shown in FIG. 1. To achieve these superior data rates 104, 105, however, requires the use of receiver structures that are far more advanced then those used in typical receivers. To illustrate, information-theoretic successive interference cancellation (IT-SIC) can improve the performance of a CDMA system to where it actually performs better than TDMA systems. While such a result is counterintuitive at first, it is noted that the performance of a TDMA system is limited by the availability of orthogonal or non-interfering time-slots. IT-SIC structures allow interference, but in a controlled manner, and shift interference cancellation to the receivers. Utilizing IT-SIC structures enables a CDMA system to achieve data rates beyond those achievable with TDMA systems, as indicated by the line 104 on the graph in FIG. 1.
There are several problems with this IT-SIC approach. First, it requires highly complex receivers. Providing complex receivers is particularly problematic in modern cellular systems, wherein receivers are expected to fit into relatively small, inexpensive terminal units with limited battery life. In addition, IT-SIC receivers must possess information regarding both their own communication channel and the communication channels of all other receivers in the system. Dissemination of such channel information in practical communication systems is highly challenging.
The problems cited above may be addressed using a technique called dirty paper coding (DPC). It is known theoretically that DPC performs at least as well as IT-SIC, and in many cases better, as illustrated by line 105 on the graph 100 in FIG. 1. Recent results have shown that DPC is an optimal communication strategy for certain multiple-input multiple-output (MIMO) PtM communication systems. In addition to providing for superior system performance, DPC has the added benefit of being a transmit-side (“pre-coding”) technique. In other words, as in traditional TDMA and FDMA systems, the burden and complexity of interference cancellation and/or prevention is dealt with in the transmitting terminal. Unlike TDMA and FDMA, however, DPC is not restricted by the limitations of orthogonal dimensions in a given signal space. As a result, DPC receivers are only required to possess detailed information pertaining to their particular communications. Furthermore, because each DPC receiver operates optimally without possessing details of transmissions intended for other receivers, DPC provides a methodology for hiding transmissions from unintended receivers, thus making it suitable to support data hiding, watermarking, and other security applications. It is noted that although DPC receivers may be somewhat more complex than conventional point-to-point receivers, they are certainly less complex than most sophisticated multi-user receivers, such as those required for IT-SIC.
The term “preceding”, as used herein, refers to the mutual coding of multiple data streams while in the transmitter in order to pre-cancel, fully or partially, any interference the data streams may cause each other; as opposed to attempting to cancel interference at individual receiving terminals post-transmission. It should be understood that pre-coding does not specifically imply that further coding steps will be performed, although further coding functions are possible.
While recent analysis of DPC has yielded significant progress in the theoretical understanding of this technique, little is understood about how to build practical communication systems with DPC, and in particular, communication systems in which communication rates tend to vary.
Accordingly, it is desirable to have a method and apparatus that utilizes rate-compatible DPC techniques to optimize system performance and improve signal quality of transmission signals in view of varying communication rates.